A variety of devices exist which may be implanted in a body to supplement or replace natural body functions. Typically, devices may be used to assist the heart in maintaining the steady pumping action needed to sustain life, to control bladder functions, to produce muscle contractions effective to combat scoliosis, to assist in countering pain-producing nerve impulses, and to control the infusion of various solutions into the body. Such devices may be implanted in patients for long periods of time and be required to maintain a selected function over this period of time while powered from a single finite source of energy, typically a lithium battery.
A long lived battery of the lithium type currently available has a total capacity of about 3.2 ampere-hours. Over a typical design lifetime for an implanted stimulating device of seven years, a maximum average current consumption of about 50 microamperes (.mu.a) can be maintained. Relatively high current consumption can be sustained for short periods of time so long as the average consumption is maintained.
In the developing generation of body function assistance devices, digital electronics are replacing the analog electronics which were originally provided. Using digital techniques, body function assistance devices may be provided which are much more versatile than the analog devices. Digital counters and storage registers combined with improved techniques for communicating information between an external device and an implanted device give such digital devices the capability of varying the output parameters to suit a variety of changing physio-logical conditions.
In spite of advances in digital electronics, a variety of specialized devices must still be manufactured since each device generally operates only in the sequence built into the device. Changing physiological needs cannot be readily accommodated without changing the implanted apparatus. In addition, the changing physiological needs of a body cannot be analyzed by implanted devices and a response generated which is interactive with the analysis.
The electronics art has developed microprocessors (MP), devices which incorporate the electronic components necessary to perform arithmetic calculations with the small size needed for implantable devices. An MP has the capability of accepting data from various body sensors, analyzing the data, and generating a response appropriate for that particular analysis. Such a device would be suitable for greatly enhancing the capabilities of implanted body function assistance devices. However, such devices have not heretofore been acceptable because of the relatively large power consumption required to operate such devices.
A typical MP using CMOS (complimentary metal oxide semiconductor) technology, having the lowest power consumption presently available, still requires up to 10 milliamperes of current when executing an operating routine. It is readily apparent that such a large current consumption would be unacceptable.
Although some techniques are known in the prior art for reducing the overall current consumption of an MP, such technique, or techniques may not generally be compatible with an implanted body function assistance device. A suitable device must execute instructions reliably and be capable of independent verification of operating instructions in order to be certain that an inadvertent output signal or a loss of output signal does not occur. Further, the device must remain capable of responding to a variety of external physiological conditions in order to maintain its usefulness.
It would also be desirable to obtain the capability of actually changing the operating routine of an implanted body function assistance device. Heretofore, various operating parameters could be changed but not the operating routine itself. However, a typical programmable device requires the use of random access memories (RAM) where the operating instructions must be retrieved from the memory and then returned to the memory, requiring considerably more energy than a read only memory (ROM) where the instructions are fixed in the memory. However, the low power ROM does not afford the capability of altering the operating routine which could be obtained with a RAM.
These, and other problems, have been solved by applicants herein where an improved body function assistance device is provided which utilizes MP technology to monitor and control the operation of the body function assistance device.